This invention relates generally to an optoelectronic modulator driver circuit, and more specifically to driver circuits for controlling and powering electro-absorption or micro-ring modulators; as well it relates to an optical transmitter comprising such driver circuit.
High speed optical transmission networks in the area of around 25-50 Gb/s often rely on the modulation of light in order to transmit data. Lasers are often used as the light source for this light data. Rather than modulating the laser itself directly a separate modulator is sometimes used. The laser is operated in continuous wave mode and the laser light is then passed into a light modulator. The modulator varies the amount or intensity of the laser light passed through it.
One such type of modulator is the electro-absorption (EA) modulator, another more recently discussed modulator type is the micro-ring modulator.
The EA modulator takes a generally laser diode light input and generates intensity-modulated light signals in accordance with applied voltage over it (drive voltage). EA modulators comprise the Franz-Keldysh (FK) type modulator, which operate based on the Franz-Keldysh effect, where even conventional semiconductors show the effect of tunneling which allows overlap of electron and hole wave functions for photon energies less than the band gap energy. FK modulators can be operated at very high speed and modulation bandwidth of tens of gigahertz can be achieved.
The other mentioned type of modulator is the micro-ring (MR) modulator or resonant micro-ring modulator, which is applicable to optical switching. It consists of a micro-ring with resonator modes, sometimes spaced by approximately 100 GHz, intended for use in wavelength-division-multiplexed (WDM) systems. This spacing allows the micro-ring to operate as a comb switch on a broadband-wavelength-parallel data stream in much the same way a smaller-diameter ring would act upon a single-channel signal. The MR modulator is based on resonant micro-ring attenuation often provided in silicon for forming a ring-resonator-based silicon modulator. When light of one or more selected resonant wavelengths of the modulator is passed through the MR modulator from an input waveguide, the light intensity builds up over multiple round-trips due to constructive interference. Because only a select few wavelengths will be at resonance within the micro-ring, it functions as a filter or modulator of the light input.
Many such diode based modulators, including the FK and MR modulators, are characterized by requiring a bias and by generating a photocurrent which is directly related to the amount of light being absorbed. Thus, at least three features are required of the electronic circuits attached to such a modulator, such as the FK or MR modulator: 1) ability to provide bias for the drive voltage, 2) ability to source and/or sink the photocurrent, and 3) ability to provide the modulation using fast signal adjustment, i.e. high frequency (or so-called AC) signals.
In known implementations these electronic driver circuits have been implemented as high speed drivers monolithically provided on an integrated circuit, IC or microchip, connected to the modulator through the high frequency path of a so called bias-T. In FIG. 1 a bias-T, indicated by the dashed box, is shown comprising two bias-T circuits, L1, C1 and L2, C2, respectively, one for the low frequency signal and one for the high frequency signal path into the modulator in question by the set voltage swing on the 50Ω driver and of the values of the capacitors C1 and C2. The necessary offset voltage provides the bias for the drive voltage on the complementary driver outputs and is provided via the inductors L1 and L2 in series with a voltage offset. High speed, such as picosecond, bias-T's are available today which can supply an active device like an optical modulator with such a bias voltage while allowing said high speed ultra-broadband signals to pass through and allow minimum signal degradation.
Bias-T's are implemented with discrete and external components because the component values needed generally are too large for integration in either the driver IC or the modulator itself. Large capacitance and/or inductance values are needed in order to reduce the cross-over frequency which distinguishes the high and low frequency paths. This is required in many high speed communication systems since the energy spectrum of the modulation signal has components at very low frequencies. Failing to couple this signal components onto the modulator will lead to distortion and degradation of the resulting frequency signal.
An EA, such as a FK modulator, and a MR modulator requires a voltage swing Vmod over the terminals thereof for driving the fast signal, and this previously entailed the use of external high-quality voltage supply. Now, in GaAs, Si based such as GE-on-Si modulators, high in-plane electric fields can be generated with moderate voltages. Furthermore, a low capacitance of these structures is particularly favorable for high speed applications. The necessary voltage swing can therefore be generated by a differential amplifier supplying a differential signal to the modulator input.
U.S. Pat. No. 7,099,596 describes an optical transmitter comprising an EA modulator and its driver circuit, where the driver circuit comprises such differential pair and an emitter follower circuit at the output stage of the driver circuit, see FIG. 2. The modulator drive voltage is herein provided by supplying voltage VCC1 over the differential pair of one branch thereof and over an emitter follower 1-4 which provides the modulator drive voltage on a single end outputted to the modulator in reverse bias. By using a single-ended output a relatively high voltage swing is needed.